Human Cripto-1 (CR-1) is a member of a larger family of structurally related proteins, namely the EGF-CFC family that includes mouse CR-1, mouse cryptic, Xenopus FRL-1, and zebrafish one-eyed pinhead (oep) (Ciccodicola et al., EMBO J., 8:1987–1991 [1090]; Dono et al., Develop., 118:1157–1168 [1993]; Shen et al., Develop., 124:429–442 [1997]; Kinoshita et al., Cell 83:621–630 [1995]; and Zhang et al., Cell 92:241–251 [1997]). These proteins are characterized by the presence of a modified EGF-like domain and by a second cysteine-rich region called CFC domain (Salomon et al., BioEssays 21:61–70 [1999]). They also share a conventional signal sequence and a hydrophobic C-terminus that is essential for membrane-anchorage by a glycosylphosphatidylinositol (GPI) moiety (Minchiotti et al., Mech. Develop., 90:133–142 [2000]). The human CR-1 and mouse CR-1 genes encode glycoproteins of 188 and 171 amino acids respectively, with molecular weights of 28 and 24 kDa, resepectively (Brandt etal., J. Biol. Chem., 269:17320–17328 [1994]). The EGF-CFC proteins perform an essential role during early vertebrate embryogenesis by promoting mesoderm formation and cell migration during gastrulation (Zhang et al., supra; Gritsman et al., Cell 97:121–132 [1999]; and Ding et al., Nature 395:702–707 [1998]). CR-1 mRNA and immunoreactive protein are expressed in several human breast cancer cell lines, in approximately 80% of human primary breast carcinomas, and in mammary tumors that arise in mice that overexpress different transgenes in the mammary gland such as c-neu, transforming growth factor α (TGFα), ini-3, polyoma middle T gene, and SV-40 large T gene (Kenney et al., Mol. Carcinogen., 15:44–56 [1996]; Qi et al., Brit. J. Cancer 69:903–910 [1994]; and Dublin et al., Int. J. Oncol., 7:617–622 [1995]). CR-1 can also be detected in the developing mouse mammary gland with different levels of expression in the virgin, pregnant, lactating and aged mammary gland (Kenney et al., Mol. Reprod. Develop., 41: 277–286 [1995]). In the virgin mammary gland, CR-1 expression is found primarily in the cap stem cells of the growing terminal end buds and CR-1 expression increases several fold in ductal epithelial cells during pregnancy, lactation, and in the aged mammary gland (Kenney et al., Mol. Reprod. Develop., 41:277–286 [1995]; and Herrington et al., J. Cell. Physiol., 131:215–226 [1997]). In addition, CR-1 can modulate the expression of milk proteins in a mouse mammary epithelial cell line (HC-11) and in primary mouse mammary explant cultures (De Santis et al., Cell Growth Different., 8:1257–1266 [1997]). However, despite the identification of CR-1 in mammary glands, its role in mammary health and disease has remained largely unknown. Thus, there remains a need in the art for methods that provide for simple detection and quantitation of Cripto-1 in various biological fluids.